Collision Analysis of the Spar Upper Module Docking

J. Marine Sci. Appl. (2014) 13: 193-199DOI: 10.1007/511804-014-1238-xCollision Analysis of the Spar Upper Module DockingYan Liu*, Liping Sun, Chunlin Wu and Guo WeiDeepwater Research Center; Harbin Engineering University, Harbin 150001, ChinaAbstract: In order to assess the possible collision effect,2 Thenonlinear finite element theory ofnumerical simulation for theand;par pladocking at the speed of 0.2 m/s was conducted by using thecollision analysis during dockingsofware ANSYS/LS-DYNA. and the time history of the ollisionAccording to the literature (Fu, 2011), the governingforce, energy absorption and structural deformation during thecollision was described. The purpose was to ensure that theequations of the nonlinear finite element are deduced, andplatform was safely put into operation. Furthermore, this paperasthe differential equation of the platform motion is presentedanalyzes different initial velocities and angles on the Von Misesstress and collision resultant force during the docking collision. Theresults of this paper showed that the docking could be conductedMa, +Cv. + Kd,=Fxu1)with higher security. The data in this paper can provide usefulreferences for the determination of the upper module's offshorehoistng scheme and pracical construction by contrasting the cllision structures; M is the mass matrix; c is the dampingnumericalsimulationresults of the parameters on the docking: matrix; K is the stiffness matrix; an is the acceleration vectorof n step; Vn is the velocity vector of n step; dn is theKeywords: spar upper module; docking; offshore lifting; collision displacement vector of n step. Using the force vector and theanalysis; spar platform; simulation analysisvector equation of excess force, the acceleration can bedrawn.Article ID: 1671-9433(2014)02-0193-07At every time-interval, when the acceleration keepsconstant, the explicit integral is calculated by using the1 Introductioncentral difference method, as illustrated in Fig.1At present, the research on the spar platform is mainlyconcentrated on its hydrodynamic performance (Hu et al,2008), while the collision analysis is simply performed forships instead of offshore plaforms (Ding et al, 2014).n-I(n-1)2(n+1)2TimesZhang (1999) used analytical and numerical methods tod,F,aanalyze collision problems between ships and floatingFig. 1 A diagram of the central difference methodplatforms or ships and bridges (Davidson et al, 2013), andsummarized how the elastic structure's effective energyThe equation derived above shows that with the knownwould be absorbed. Mazaheri et al. (2008) and Zeng et al. coordinate vector and acceleration vector at In and velocity(2011) analyzed ollision response between ships and the vector at In+1)/:, the coordinate vector x(nt) at th+t1 can bedeep-water semi-submersible offshore platform usedderived. Likewise, the coordinates, velocity and accelerationANSYS and MSC/DYTRAN software respectively. Ding etvector of each time node can be obtained. The recursiveal. (2011) have conducted studies on the ollision process of methodsjack-up platform docking. Offshore installation is anexplicit algorithms. The explicit algorithms do not involveimportant part of the deep sea project, and is the last matrix decompositon and solving during the computingsignificant engineering done before the platform is put intoprocess, but use of circular computations of the explicitoperation, and plays a decisive role in successful completionintegral at every time interval instead.of the petroleum mining project (Wei, 2012). Therefore it isDuring the docking of the spar upper module, thenecessary to analyze the collision during the process ofcollision is a complex process of nonlinear dynamicistallation while considering dfferent initial velocities and response in which the platfrm's material may exceedangles on the Von Mises stress and collision resultant force.beyond the elastic stage and enter into the plasticdeformation stage. Research shows that the yield and tensileReceived date: 2013-07-29.strength of the ma中国煤化工increase ofAccepted date: 2013-10-15.high strain rates.CNMH Glects of theFoundation item: Supported by the Programme of Introducing Talents ofstrain-rate, sensitivYHmny ' vial w u simulation*Corresponding author Email: liuyan 1989 1018@ 163.comresults.◎Harbin Engineering University and Springer- Verlag Berlin Heidelberg 2014Using the nonlinear finite element analysis program19Yan Lil, et al. Collision Analysis of Spar the Upper Module DockingANSYS/LS-DYNA, the plastic kinematic model is adopted,which uses the constitution equation of Cowper-Symonds,providing accordant data with the experiment. The equationis shown as follows: .o,=1+(言)(σ。+ BE,e;")(2)(a) Finite element model of the main hull and upper modulewhere σy is the dynamic yield stress when the plastic strainrateis E;σ is the corresponding static yield stress; D and pare the strain rate coefficients obtained by the tests. For thegeneral steel shown in the literature (Wang, 2001), D=40.4,p=5; ep" is the effective plastic stain; β is thehardening parameter; Ep is the plastic hardening modulus,which is obtained by E,=EEJE-En (Lin e1 al, 2012).The constitution equation (2) and governing equation of(b) A quarter module of the main hull structurethe nonlinear finite element (1) compose all the solvingFig.2 Finite element model of ollision structurescollision problem equations.3 Collision simulation analysis of the upper3.2 Stress analysis during collisionDuring the docking of the upper module, due to wavemodule dockingeffects, the floating crane would heave with some verticalvelocity components, and so did the spar, therefore, the3.1 ModelingIn this paper, the Liwan 3- lspar platform, the secondcollision velocity should not be decreasing in the upperWith the assumption that the collision velocity isgeneration truss platform having 7 decks consisting of a0.2 m/s, the time duration to compute is taken as 1.5s whilehard tank, is selected as the calculating model with the mainthe gap between the main hull and the upper module is 0.1mdimensions shown in Table 1.from the very beginning, and the module and main hull areTable 1 Main dimensions of the Liwan spar platformwhere the vertical direct impact occurs (Liu and Li, 2013).NameOffsetFig. 3 and Fig. 4 represent the Von Mises stress nephogramof the upper module and the main hull at the two differentOverall length/m165moments.Hard tank diameter/m27.4Hard tank length/m)0Displacement/t40 249Draft/m150Consideration is given to the fact that the full structure(a) Whole structure of sparmodeling would cost plenty of time and make thecomputation almost impossible since tremendous finiteelements exist. On the other hand, the collisions mostly川ioccurred in the areas close to the sixth and seventh decks.Therefore in this paper, the model is appropriatelysimplified based on the literature (Wang, 2001)，and only thestructures around the sixth and seventh decks were built up,which consisted of shell plates, vertical sifeners, horizontal(b) Internal structure of main hullwebs, circular frames, decks, vertical bulkhead, center well,four pillars and 16 bracings.It was considered that the beam elements could not reflectthe collision force and structural deformation very well,although there are a large number of T bars and angle steelexists everywhere in the platform, as well as bracing中国煤化工elements in the primary region of the collision. All theTYHCNMH G .structure members, therefore, were modeled by the 163Fig. 3 Von Mises stress nephogram for each structureexplicit shell elements. The finite element model totaledmember att = 0.53025s62 299 nodes and 62 175 elements, as shown in Fig. 2.Journal of Marine Science and Application (2014) 13: 193-19995It can be seen that the collision force rapidly climbed upto the peak value 0.36s right after the collision happened,and afterward fluctuated for a period of time until goingdown to zero (Hu et al, 2013). The results indicate that thecollision would repeat from time to time since it is difficultto make the four pillars touch the main hull simultaneously,(a) Whole structure of sparthus the collision would not be under the perfect condition.3.4 Energy transformationPrior to the collision, the total energy storage in the uppermodule and main hull behaved in the form of kinetic energywhich was E=1/2mv=52.36kJ. At the beginning of thecollision, the kinetic energy rapidly transformed into internal(b) Internal structure of main hulland other forms of energy, e.g. frictional energy, hourglassenergy, etc, as ilustrated in Fig. 6.6￥10'. Kinetic Energy” Total Energys4(c) Upper module decksFig. 4 Von Mises stress nephogram for each structuremember att = 0.757 50s2|It can be seen from the figures that the Von Mises stressn the upper module along the four pillars and bracingsspreads upward from the contacting area, and the stress in0.20.40.60.81.01.21.41.6the main hull diffuses to the periphery. The stress in theFig. 6 Energy transformation curvesinternal structure of the platform spreads along the verticalangle of the steel downwards and diffuses to the periphery.During the whole docking, at the moment of t=0.76s, theAs shown in the figure above, during the docking, themaximum Von Mises stress of the collision appears at thekinetic energy of the upper module will transform to internalelement numbered 49 682, which is the shell plate at theenergy in the upper module and main hull of the spar untilcontact with the main pillar of the upper module, with thedeclining to zero at 1=0.65 s. Since then a lttle rebound wasvalue of 22.70MPa, less than the yield stress. This meansobserved due to some internal energy that was retransformedthat no plastic deformation occurs in the platform. It isto kinetic energy again. The computed result indicates thatnoteworthy that the stress concentration region appears atthe kinetic energy of the whole collision mainly transformedthe geometrical center of the deck along the stress of theto internal energy, which is 4.53x104 J, accounting fcupper module spreading.86.52% of the total energy stored in all the structures.3.3 Collision force4 Effects of collision velocities on collisionThe curve of the collision force in time history is shownIt is thought that the motions of both the upper modulein Fig. 5. .9.0X10^and spar are mutually independent during docking, althoughthe upper module is always controlled, and the collisionvelocity tends to vary with the wind, waves, current andeven operational errors. Therefore, it is of great significanceto investigate the effects of cllision velocities.With this paper, a set of collision velocities werepresumed to simulate the effects of different velocities。3.0during the process of collision.st4.1 Stress results f中国煤化工For simulationCN MH G"n velocities.5一 0.6 0.7 0.8 0.9were selected as shYH~ ... un duration tocompute was taken as 1.5s， the gap between the upperFig. 5 Collision force in time historymodule and the main hull was 0.1 m prior to collision, and19Yan Lil, et al. Collision Analysis of Spar the Upper Module Dockingthe module and main hull were assumed to be where the collision velocity is under 1 m/s, the plastic deformation willvertical direct impact occurred.not appear in the upper module or the main hull structures.Table 2 Different collision velocities during dockingThe results also reveal that the maximum stress appears atthe contacting zones such as the main decks of the main hullNameVelocity/(m-s)and the collision region of the main pillars and someSchemel0.1transitions of the supporting structures such as the joints ofScheme2).2the main pillars and the four bracings and the vertical angleScheme30.3steel of the main hull.Scheme40.4Scheme5).54.2 Collision force for different velocitiesThe maximum collision force is presented in Table 4 andScheme60.6plotted in Fig. 8. It can be observed from Table 4 and Fig. 8Scheme7).7Scheme80.8the force of the structures increase as the velocities increase.Scheme9).9The time of the maximum force occurrence is not inScheme101.0accordance with the maximum stress though which for allschemes, is less than the yielding stress. The maximumThe maximum stress for each scheme is presented inforce in Scheme10 is greatest and the maximum stress thatTable 3, in which the maximum stress for Scheme 1 andthe cable can bear should be considered during the docking.Scheme 5 occurred at the joints of the bracing and decks ofTable 4 Max. forces for different velocitiesthe upper module while the other schemes occurred at thecollision zones of the decks of the main hull. The maximumMax force/kNTime/sstress for each scheme is shown in Fig. 7.4.20.7107.90.360Table 3 Stress results for different velocities10.30.240Max stress/MPa13.30.18515.001.212015.80.15022.700.757518.80.12534.890.606020.80.11047.740.454524.40.10060.540.378827.30.09089.440.3030Scheme 1030.40.085113.700.2270170.500.1515X10°.0223.50275.800.07563.(s 10°平2.5重1.s0.2 0.4 0.6 0.8 1.001.Velocity/(m●s~")Fig. 8 Max force against different velocities0.0.2 -“ 0.40.60.81.05 Effects of collision anglesFig. 7 Max stress for different velocitiesDuring the docking, the swinging motion of the upperIt can be seen in Table 3 and Fig. 7 that the time of the module appears ar中国煤化工ision anglescollision occurrence advances in order and the stress rises asto emerge. Althougcables fixed,the collision velocity increases. The maximum value occurs the small angle-YHCN MH Gieremainsat 1m/s and reaches 275.80MPa, which is still less than the unavoidable. It is, therefore, necessary to study the efcts ofyield stress of the material. This means that as long as the collision angles on the collision.Journal of Marine Science and Application (2014) 13: 193-19997In this paper, a set of collision angles at different are four different velocities from 0.2 m/s to 0.8 m/s.velocities is simulated to study the effects of different anglesThe collision angles refer to the angles of the horizontalon the collision. There are four different collision angles, plane. The specific collision location of the main hull andwhich are 0"，1°, 2° and 3°. At each collision angels, there upper module is shown in Fig.9 at different collision angles.71.(a) At the angle of 0°(b) At the angle of 1°7(c) At the angle of 2°(d) At the angle of 3°Fig. 9 The location relationship of the upper module and main hull at four angles5.1 Stress results at different anglesfindings imply that the direct docking method should beAt different angles, the maximum Von Mises stress is used to minimize the damage of the collision.shown in Table 5, and the maximum stress of all groups10- 0.2 m/soccurred in the collision zones of the decks of the main hull.The plots of the maximum stress in each group are shown in上06m/s+ 0.8 m/sFig.10. It can be demonstrated from Table 5 and Fig.10 that1.0 m/sat a certain velocity, the maximum Von Mises stress riseswith the increasing of the collision angles, and appears at theshell of the main decks. When the angle reaches 2° at avelocity of 1 m/s and 3° at a velocity of 0.6m/s, 0.8 m/s and1.0m/s, the maximum stress is 425.80 MPa, 372.50 MPa,415.10MPa and 449.90MPa respectively, which exceeds theyielding stress and therefore the plastic transformation0.5~ 1.0152.0 2.5 3.0would appear on the structures. It is summarized that atAngle/(°)small collision angles, the maximumFig. 10 Max stress curves at different angleswould increase simply with the increasing of the angles,which is minimal when direct collision occurs. TheseTable 5 Max stress of collision at different anglesCollisionMax stress/MPaVelocity/(ms l)Group1(0°)Group2(1°)Group3(2°)Group4(3°)0.222.7047.8394.36119.800.444.74102.10153.10252.600.89.44171.50233.30372.50170.50201.10310.20415.10 .1.275.80346.40425.80449.90中国煤化工5.2 Collision force at different anglesFig.11.The maximum collision force of the upper module andIt can be seen 1.MHCN MHGat the samemain hull at different angles is presented in Table 6. The velocity, the maximum collision force decreases as thplots of the maximum force in each group are shown in increasing of angles. From the Fig. l1, however, the results19Yan Liul, et al. Collision Analysis of Spar the Upper Module Dockingreveal that it creates the risk of slanting, which will cause maximum stress that the cable can bear ought to behuge damage. Upon the study above, it could conclude thatconsidered during the docking.the direct docking method should be taken and theTable 6 Max force at different anglesMax force/kNCollisionVelocity(ms ) Group1(0)Group2(1°)Group3(2°)Group4(3°)0.27.904.211.410.413.309.845.220.18.8016.309.347.820.824.4021.6015.1012.201.030.4027.2021.0017.20offshore hoisting scheme and practical construction. So far-0.2 m/sthe research on collisions mainly focus on ships and+ 0.4 m/s-0.6 m/splatforms, it is therefore necessary to study the tests for; 2.5-0.8 m/shoisting, which is also the following key content.s 2.0- 1.0 m/sReferencesY 1.0-Davidson M，Chung J, Bollmann H, Consolazio G (2013).Computing the responses of bridges subject to vessel collisionloading using dynamic analysis. Bridge Structures -Assessment,Design & Construction, 36(9), 169- 183.0.5 1.0152.025 3.0Ding Hongyan, Zhu Qi, Zhang Puyang (2014). Dynamicsimulation on collision eetween ship and offshore wind turbine.Fig. 11 Max force curves at different anglesTransantions of Tianjin University, 20(1), 1-6.Ding Zhongjun, Liu Baohua, Xiu Zongxiang, Tian Haiqing (2011).6 ConclusionsDocking collision analysis of jack-up platform based onpenalty function method. 2011 International Conference onIn this paper, the Liwan 3-1 spar platform was selected asElectric Technology and Civil Engineering, Lushan, China,the calculation model. 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